A gas fuel injector (3) supplies gas fuel from a gas cylinder (1) into a measuring section thin tube (5), and a pressure measuring device (4) measures pressure variation in the tube through a small hole provided in the thin tube (5). On the downstream side of the measuring section thin tube (5), an extension thin tube (6) for removing the influence of reflected wave is provided. At the downstream side end of the extension thin tube, a back pressure valve (13) for increasing the pressure in the tube uniformly and bringing it close to the actual environment in the engine cylinder is provided. Furthermore, a tapered nozzle is arranged in the measuring section thin tube (5). Pressure measured by the pressure measuring device (4) is converted into the flow rate in the thin tube according to a predetermined calculation expression. Consequently, instantaneous flow rate of gas fuel being ejected from the gas fuel injector (3) can be measured.